Acoustic transducers or transponders are used for transducing acoustic (sound) energy with electrical energy. This may be useful, for example, for producing sound in response to electrical signals, as in a loudspeaker, or for producing electrical signals in response to sound energy, as in a microphone. In this context, the term sound also means ultrasound. The design of an acoustic transducer is strongly impacted by the fluid medium for which it is intended, and whether it is intended for producing sound energy in the medium, or extracting energy therefrom. When electrical energy is applied to the acoustic transducer for coupling to the fluid medium, the transducer must be strongly coupled to the fluid, otherwise the electrical energy will not be transferred to the fluid (will be reflected to or remain in the electrical source), or will be absorbed in the transducer itself, thereby causing heating. Strong coupling to the medium generally means a relatively large aperture, so that significant amounts of the fluid may be moved in response to the input electrical energy, and the structure must be sufficiently large to handle the heat energy and forces involved in the transduction. Acoustic transducers intended for sensing or picking up sounds, on the other hand, may be small, as they are unlikely to absorb so much energy from the medium that they heat up, and the relatively small electrical signals which are produced can generally be amplified to useful levels. A further advantage of physically small transducers is that they tend to have relatively good frequency response, by comparison with larger transducers, because their mechanical resonances occur at higher frequencies than those of larger transducers, and they therefore have a broader frequency range over which the amplitude response of the transducer is flat.
Transducers for underwater purposes such as sonar are often operated in both a transmission mode, and, at a different time, in a reception mode. The requirements of the transmission mode tend to dominate the design of such transducers. U.S. Pat. No. 5,239,518, issued Aug. 24, 1993 in the name of Kazmar, describes one such sonar transducer, therein termed a "projector." The Kazmar transducer includes an electrostrictive or piezoelectric material, which responds to electrical signals to produce corresponding acoustic signals, and which also transduces in the other direction, producing electrical signals in response to acoustic energy.
The velocity of sound signals depends upon the density of the medium; the velocity of sound in air is about 1100 ft/sec., in water about 4800 ft/sec., and in steel about 16000 ft/sec. Since the wavelength in a medium at a given frequency is directly related to the velocity of propagation, the wavelength in water at any given frequency is much larger than in air. Consequently, a given structure is smaller, in terms of wavelengths, in water than in air. Therefore, structures such as acoustic transducers tend to be relatively small in terms of wavelength when immersed in their fluid medium, water. A concomitant of small size in terms of wavelength is isotropy or nondirectionality of the response; a transducer which is very small in terms of wavelengths effectively appears to be a point source, and transduces in a nondirectional or omnidirectional manner.
Directional transduction is desirable for many reasons. For example, when using a transducer to listen to distant sound sources, a directional "beam" tends to reduce the influence of noise originating from other directions. When transmitting acoustic energy toward the location of an object to be detected by observation of the acoustic reflection, a directional transmission "beam" concentrates the available energy toward the object, making it more likely that sufficient energy strikes the object that its reflection can be detected. However, as mentioned, an acoustic transducer tends to be small in terms of wavelength, and to provide omnidirectional transduction.
A well-known method for increasing acoustic directionality is to arrange a plurality of individual transducers in an array. For example, long "line" arrays of acoustic transducers may be spaced along a cable, and towed behind a ship performing undersea examination. The acoustic transducers are energized simultaneously in a transmit mode, so that they act in concert, with the result that the effective dimension of the transmitting transducer is established by the length of the cable, rather than the dimension of an individual transducer. This enables a directional beam to be produced, which in the case of the described towed array is a "fan" beam orthogonal to the cable's length. The same towed array, operated as a receive transducer, combines all of the received signals without relative delays or phase shifts, and achieves a "receive beam" corresponding to the abovementioned fan beam.
Other types of arrays are known. An April, 1987 report prepared for Naval Underwater Systems Center, New London, Conn., under contract NICRAD-85-NUSC-022 describes an array of twenty-one transducers in the form of a right circular cylinder, which is advantageous because of its symmetry in the horizontal plane, and the resulting 360-degree azimuth coverage. The diameter and height of the described cylindrical array are about one wavelength. The elements were driven with relative time delays for phasing to a plane.
An arrangement of transducers on the surface of a sphere is described in U.S. Pat. No. 5,377,166, issued Dec. 27, 1994 in the name of Kuhn. This arrangement has the advantage that a directional beam can be pointed generally in any direction in three-dimensional space; in one embodiment it includes twelve transducers located at the vertices of an icosahedron, and in another embodiment it includes twenty transducers located at the vertices of a dodecahedron. These regular polyhedrons have the advantage that each transducer is equidistant from its adjacent transducer, and the mutual coupling effects on the transducers are the same, so their "radiation" impedance is the same from transducer to transducer. In one embodiment of the arrangement described in the abovementioned Kuhn patent, the icosahedral array is concentric with the dodecahedral array. The element-to-element spacing of the transducers in both arrays is selected to lie between .lambda./3 and 2.lambda./3, to prevent unwanted peaks in the array response.
Each of the transducers of the Kuhn patent has a maximum dimension of less than one acoustic wavelength in the medium, as a result of which the transducers tend to be isotropic, meaning that each one radiates equally well in all directions, with the further result that the directivity or directional gain of the array is entirely due to the array factor and the overall dimensions of the array, rather than to the characteristics of the transducers themselves. The minimum beamwidths achieved by Kuhn are described in the '166 patent as being about 30.degree.. While the Kuhn arrangement is satisfactory, there may be cases in which it is desired to have narrower or more selective beams, in which case greater directive gain must be achieved, which in turn requires a larger array aperture. The maximum gain of a Kuhn arrangement is determined, in part, by its effective aperture, which may be estimated by considering that the inter-element spacing along the surface of the sphere is a maximum of about 2.lambda./3, which makes the maximum diameter of the dodecahedral sphere about two wavelengths, and the maximum diameter of the icosahedral array is smaller. Thus, to achieve more selective beams, more directional gain must be provided than that which can be achieved by the dodecahedral arrangement of the '166 patent. The larger aperture requires a larger "diameter" of the sphere of transducers. The dodecahedron, however, is the largest of the classical regular polyhedrons. Consequently, some structure other than a dodecahedron must be used to define the array. Improved array configurations are desirable.